Subthreshold stimuli trains may inhibit the response to a subsequent suprathreshold stimulus; the inhibitory effect decreases or even disappears when the subthreshold stimuli are delivered apart from the suprathreshold stimulus. The purpose of this study is to analyse, in atrial epicardium, the influence of different electrode configurations upon the inhibitory effect of subthreshold stimuli trains delivered at a short distance from the explored zone. In 18 anaesthetized dogs, the basal atrial effective refractory period was determined using an epicardial unipolar cathodal electrode. A second determination was performed when a subthreshold stimuli train (1 ms cathodal pulses, 800 Hz, and intensity 10% less than the diastolic threshold of the train) was delivered prior to the extrastimulus through: (a) 8 electrodes equidistant (4.5 mm) from the central test electrode (Group 1, n = 5); (b) a 1 mm wide ring electrode, 2.5 mm from the central test electrode (Group 2, n = 7); and (c) a 3.5 mm wide ring electrode, 2.5 mm from the central test electrode (Group 3, n = 6). A third determination was performed in all experiments delivering both the subthreshold stimuli train and the extrastimulus through the same central test electrode. Atrial effective refractory period did not vary significantly with respect to the basal values in Group 1 (141 +/- 22 vs 142 +/- 25 ms; ns) though it increased in Group 2 (168 +/- 27 vs 142 +/- 18 ms; P < 0.01) and in Group 3 (160 +/- 20 vs 133 +/- 25 ms; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)